VirtualProtect
The VirtualProtect
function changes the access protection on a region of committed pages in the
virtual address space of the calling process. This function differs from VirtualProtectEx, which changes the access
protection of any process.
BOOL VirtualProtect(
|
LPVOID lpAddress, |
// address of
region of committed pages |
|
DWORD dwSize, |
// size of the
region |
|
DWORD flNewProtect, |
// desired access
protection |
|
PDWORD lpflOldProtect |
// address of
variable to get old protection |
|
); |
|
Parameters
lpAddress
Points to the
base address of the region of pages whose access protection attributes are to
be changed.
All pages in
the specified region must have been allocated in a single call to the VirtualAlloc or VirtualAllocEx function. The pages cannot
span adjacent regions that were allocated by separate calls to VirtualAlloc
or VirtualAllocEx.
dwSize
Specifies the
size, in bytes, of the region whose access protection attributes are to be
changed. The region of affected pages includes all pages containing one or more
bytes in the range from the lpAddress parameter to (lpAddress+dwSize).
This means that a 2-byte range straddling a page boundary causes the protection
attributes of both pages to be changed.
flNewProtect
Specifies the
new access protection. You can specify any one of the following flags, along
with the PAGE_GUARD and PAGE_NOCACHE protection modifier flags, as necessary:
|
Value |
Meaning |
|
PAGE_READONLY |
Enables
read access to the committed region of pages. An attempt to write to the
committed region results in an access violation. If the system differentiates
between read-only access and execute access, an attempt to execute code in
the committed region results in an access violation. |
|
PAGE_READWRITE |
Enables
both read and write access to the committed region of pages. |
|
PAGE_WRITECOPY |
Gives
copy-on-write access to the committed region of pages. |
|
PAGE_EXECUTE |
Enables
execute access to the committed region of pages. An attempt to read or write
to the committed region results in an access violation. |
|
PAGE_EXECUTE_READ |
Enables
execute and read access to the committed region of pages. An attempt to write
to the committed region results in an access violation. |
|
PAGE_EXECUTE_READWRITE |
Enables
execute, read, and write access to the committed region of pages. |
|
PAGE_EXECUTE_WRITECOPY |
Enables
execute, read, and write access to the committed region of pages. The pages
are shared read-on-write and copy-on-write. |
|
PAGE_GUARD |
Pages in
the region become guard pages. Any attempt to access a guard page causes the
operating system to raise a STATUS_GUARD_PAGE exception and turn off the
guard page status. Guard pages thus act as a one-shot access alarm. The
PAGE_GUARD flag is a page protection modifier. An application uses it with
one of the other page protection flags, with one exception: it cannot be used
with PAGE_NOACCESS. When an access attempt leads the operating system to turn
off guard page status, the underlying page protection takes over. If a guard
page exception occurs during a system service, the service typically returns
a failure status indicator. |
|
PAGE_NOACCESS |
Disables
all access to the committed region of pages. An attempt to read from, write
to, or execute in the committed region results in an access violation
exception, called a general protection (GP) fault. |
|
PAGE_NOCACHE |
Allows no
caching of the committed regions of pages. The hardware attributes for the
physical memory should be specified as no
cache. This is not recommended for general usage. It is
useful for device drivers; for example, mapping a video frame buffer with no
caching. This flag is a page protection modifier, only valid when used with
one of the page protections other than PAGE_NOACCESS. |
lpflOldProtect
Points to a
variable that the function sets to the previous access protection value of the
first page in the specified region of pages. If this parameter is NULL or does
not point to a valid variable, the function fails.
Return Values
If the
function succeeds, the return value is nonzero.
If the
function fails, the return value is zero. To get extended error information,
call GetLastError.
Remarks
You can set
the access protection value on committed pages only. If the state of any page
in the specified region is not committed, the function fails and returns
without modifying the access protection of any pages in the specified region.
The VirtualProtect
function changes the access protection of memory in the calling process, and the
VirtualProtectEx function changes the access protection of memory in a
specified process.
The
PAGE_GUARD protection modifier flag establishes guard pages. Guard pages act as
one-shot access alarms. See Guard Pages for further discussion of guard pages.
See Also